Known electron beam optical columns used in electron beam lithography systems employ an electron beam spot focused onto the target (either a reticle, mask, wafer or the like) which is either circular (Gaussian) or shaped. In the case of circular spots, they are generated by the demagnified gun crossover, are fixed in shape and spot positioning is continuous. In the case of shaped spots, the demagnified image of an aperture (typically of square shape), is used and the spot positioning is in increments of the spot size. For an example of this type of system, reference is made to U.S. Pat. No. 4,243,866 issued Jan. 6, 1981 for a "Method and Apparatus for Performing a Variable Size Electron Beam".
Using a shaped electron beam spot lithography system, a chip circuit pattern on a target surface can be generated significantly faster than with a Gaussian spot system because with the shaped spot system many pixels are written simutaneously. In the past, the type of patterns which could be written by a shaped spot system were limited to feature sizes in increments of the basic shaped spot size and to orthogonal patterns, otherwise overlapping had to be applied which would degrade the quality of the written pattern due to a so-called "blooming effect".
In order to make the shaped-spot concept more adaptable to a wider range of circuit patterns, lithography systems using variable spot-shaping have been successfully introduced such as those described in a publication entitled "Design of a Variable-Aperture Projection and Scanning System for Electron Beam"--E. Goto, T. Soma and M. Idesawa reported in the Journal of Vacuum Science Technology-- 15(3), May/June 1978, pages 883-886. Also reported in the same publication is an article entitled "Variable Spot Shaping for Electron Beam Lithography" by H. C. Pfeiffer appearing on pages 887-890; an article entitled "Double-Aperture Method of Producing Variably Shaped Writing Spots for Electron Beam Lithography" by M. G. R. Thomson, R. J. Collier and D. R. Herriott on pages 891-895 and an article entitled "Dynamic Beam Shaping" by J. Trotel on pages 872-873. A variable spot-shaping system also is described in the above noted U.S. Pat. No. 4,243,866. All of these known systems are based on the principal that one spot-shaping aperture, typically of square shape, is electron-optically imaged onto a second aperture. The image of the first aperture is offset by deflection means so that it overlaps a part of the second aperture. The resulting composite image is somewhat truncated and is demagnified to form a focused spot of variable shape at the target surface.
All of the known variable-shaped spot electron beam systems mentioned above use magnetic lenses and operate in the so-called Koehler illumination scheme. This scheme was originally devised in 1896 for use as the illuminating condenser in a light microscope to separately control the illuminating semi-angle and the field. The Koehler illumination scheme is characterised in an electron beam optical system by using the electron gun crossover as the source which is imaged by a condenser lens (and typically a further demagnifying lens) into the object principal plane of the final objective lens and determines the maximum semi-angle in the pencil-like electron beam forming the finally imaged variably-shaped electron beam spot at the target plane.
The use of magnetic lens in known variably-shaped spot electron beam optical systems is motivated by the belief that they exhibit smaller optical aberations than electrostatic lens with the same focal length. However, magnetic lens have the disadvantage of rotating the image. When the image of the first spot-shaping aperture on the second spot shaping aperture is focused and has the correct (desired) size, it usually is not oriented correctly. Thus, some mechanical means must be and is provided in these known magnetic lens systems to rotate the spot-shaping apertures from outside the vacuum column to correct for the desired orientation. This increases the difficulty of setting up a variably shaped spot system considerably and also makes the design of the system more complicated and expensive.
A second and comparably severe disadvantage of magnetic lenses is their large size which makes them unsuitable for any application in which space is at a premimum. One such use would be in a multiple-channel electron beam lithography system such as that described in copending U.S. patent application Ser. No. 749,796, filed June 28, 1985 concurrently with this application entitled "Multiple Channel Electron Beam Optical Column Lithography System and Method of Operation", Kenneth J. Harte--inventor, and assigned to the Control Data Corporation. It should be noted, however, that in the known variably-shaped spot electron beam lithography systems using magnetic lens assemblies, the deflection means typically are electrostatic because of the inherently faster response of electrostatic deflectors. To obtain practical thru-put in such electron beam lithography systems, the shaping deflection has to occur within the time the spot is repositioned, typically taking 100 nanoseconds or less, to insure reasonable thru-put for the microcircuit pattern writing facility.